Accurate power detection circuit for use in a power amplifier

ABSTRACT

A power amplifier circuit for amplifying an input signal includes an amplifying transistor and a power detection circuit. The power detection circuit includes a circuit for generating a signal which is directly proportional to the power level in the amplifying transistor. This may-be accomplished by generating a voltage proportional to the square of a current in the amplifying transistor and then averaging that voltage. In this manner, a more accurate indication of the power level in the amplifying transistor is obtained.

BACKGROUND OF THE INVENTION

The invention is in the field of transistor amplifier circuits, andrelates more particularly to power amplifier circuits and powerdetection circuits for such power amplifier circuits.

Amplifiers of this general type are frequently used in high-frequency RFamplifiers, such as those used in wireless communications apparatus, aswell as in audio amplifiers and other applications. In such amplifiers,particularly when used in RF applications, it is desirable to provide anindication of the power in the output stage.

In the prior art, this function is typically accomplished by one of twotechniques. First, a directional coupler or voltage divider may beprovided at the output of the power amplifier to sense a fraction of thesignal applied to the load, and this sample signal is then used toindicate the power in the load. The second technique used in the priorart generates a sample of the current in the power amplifier by using asmaller transistor in parallel with the power transistor. The samplecurrent is then fed off-chip, where it is averaged and used to providean approximate indication of the power in the load.

However, there are various drawbacks inherent in the prior-artsolutions. Thus, for example, directional couplers are bulky and lossy,resistive dividers are inefficient, and using a current sample as ameasure of output power is relatively inaccurate.

Accordingly, it would be desirable to have a power detection circuit fora power amplifier circuit which is compact, efficient, easy toimplement, and more accurate than prior-art power detection circuits.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a powerdetection circuit for use in a power amplifier circuit which is compactand efficient in design, easy to implement, and which provides a moreaccurate indication of amplifier power than prior-art circuits.

In accordance with the invention, these objects are achieved by a newpower detection circuit for use in a power amplifier circuit having anamplifying transistor, in which the power detection circuit includes acircuit for generating a signal which is directly proportional to apower level in the amplifying transistor.

In a preferred embodiment of the invention, this is done by providing acircuit for generating a voltage proportional to the square of a currentin the amplifying transistor and averaging this voltage to provide anindication of this power level in the amplifying transistor.

In a further preferred embodiment of the invention, a wirelesscommunications device having an RF power amplifier circuit is providedwith a power detection circuit for generating a signal which is directlyproportional to the power level in the amplifying transistor.

In further preferred embodiments of the invention, the power detectioncircuit may be implemented using a simple MOS transistor configuration,or a translinear circuit using bipolar transistors which is more complexthan the MOS circuit but which provides greater accuracy.

A power detection circuit in accordance with the present inventionoffers a significant improvement in that a particularly advantageouscombination of features, including a compact and efficient design, andmore accurate performance, can be obtained in an economical andeasily-implemented configuration.

These and other aspects of the invention will be apparent from andelucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWING

The invention may be more completely understood with reference to thefollowing description, to be read in conjunction with the accompanyingdrawing, in which:

FIG. 1 shows a simplified partially-block and partiallyschematic diagramof a power amplifier circuit having a power detection circuit inaccordance with the invention;

FIG. 2 shows a simplified schematic diagram of a power detection circuitin accordance with a first embodiment of the invention; and

FIG. 3 shows a schematic diagram of a power detection circuit inaccordance with a second embodiment of the invention.

In the drawing, like reference numerals are generally used to designatelike components.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A power amplifier circuit 10 for amplifying an input signal V_(IN) at aninput terminal 12 is shown in simplified partially-block andpartially-schematic form in FIG. 1. The power amplifier circuit includesan amplifying transistor 14 for amplifying the input signal V_(IN)provided to the base of the transistor 14 from the input terminal 12through an input matching network 16. It should be noted that the poweramplifier circuit may be used in various applications, such as audioamplification, or RF amplification in such areas as wirelesscommunication and internet connection, so that the input signal V_(IN)may be either an audio or RF input signal. The power amplifyingtransistor 14 is biased from the power supply voltage V_(CC) by a biasnetwork 18. It is to be understood that circuit portions such as inputmatching network 16 and bias network 18 which are shown in block-diagramform can be implemented by various known circuit configurations familiarto those of ordinary skill in this art, and are accordingly notdescribed in further detail herein.

The output circuit of power amplifier 10 is shown in simplified form asincluding a collector resistor 20 and an output matching network 22 forproviding an output voltage V_(OUT) at output terminal 24 to supplypower to a load symbolically illustrated by resistor 26.

In accordance with the invention, a proportional sample of the currentin the power amplifier transistor 14 is obtained by using a smallertransistor 28 having its base-emitter circuit connected in parallel withthat of the power amplifying transistor 14. However, unlike prior-artcircuits, which utilize the output of transistor 28 directly to obtain arelatively inaccurate measure of power, the present invention uses asquaring and averaging circuit 30, having its input coupled to theoutput of transistor 28, in order to generate a voltage which isproportional to the square of the current in sensing transistor 28, andthen averaging this voltage. Circuit 30 generates an output signalV_(DET) at terminal 32 which is directly proportional to the power levelin the amplifying transistor 14 and which is a substantially moreaccurate representation of this power level than can be obtained usingthe prior-art technique, since the power level is proportional to themean square of the current level in the amplifying transistor. Thistechnique permits the implementation of a power detection circuit whichcan be integrated on the same die as the power amplifier circuit andwhich can generate a voltage which is an accurate representation of thepower in the amplifying transistor in a relatively simple manner.

A particularly simple and compact MOS implementation of the squaring andaveraging circuit 30 is shown in FIG. 2. In this figure, sensetransistor 28 has a base connection (as shown by the vertical dashedline) to the base of transistor 14, as shown more fully in FIG. 1, andhas its collector coupled to V_(CC) by a series connection of a resistor34 and a diode-connected PMOS 36. The collector of sense transistor 28is also coupled to the gate of a second PMOS transistor 38, withtransistor 36 serving to generate a threshold voltage for transistor 38and the current through resistor 34 generating a voltage applied to thegate of transistor 38 to generate a squared current in transistor 38.This squared current is passed through resistor 40 to generate adetector output voltage V_(DET) at terminal 32 which is directlyproportional to the power level in the amplifying transistor. In orderto provide the averaging function in circuit 30, a capacitor 42 isprovided in parallel with resistor 40 to form a simple RC averagingcircuit, so that the detector output voltage V_(DET) is an averagedsignal which accurately represents the power level in the amplifyingtransistor. It is to be understood, however, that other types of knownaveraging circuits may be employed.

A second embodiment of the power detection circuit, using a bipolarimplementation with a translinear circuit, is shown in FIG. 3. Thisembodiment is less compact and more complex than the embodiment of FIG.2, but offers the additional advantage of still greater accuracy.

In this embodiment, sense transistor 28 has its base connected as inFIGS. 1 and 2, and has its collector coupled to a current mirrorcomposed of bipolar transistors 44 and 46 connected in a conventionalcurrent-mirror configuration. The collector of transistor 46 isconnected both to a series connection of diodes 48 and 50 and to thebase of transistor 52. The emitter of transistor 52 is connected to thebase of transistor 54 and to the collector of transistor 56, which, incombination with transistor 58 and current source 60, forms a secondcurrent mirror. Within the circuit as so far described, diodes 48 and 50and transistors 52 and 54 form a translinear circuit configuration inwhich the sum of the voltages across the two diodes is equal to the sumof the voltages across the two base-emitter junctions of transistors 52and 54, and in which the product of the currents in the two transistorsis equal to the product of the current in the two diodes. This resultsin the current in transistor 54 being proportional to the square of thecurrent in sense transistor 28 divided by the current in transistor 52.Since the current in transistor 52 is substantially equal to the currentprovided by current source 60, the circuit output can be made moreaccurate, for example by selecting the value of current source 60 tocompensate for temperature variations in the circuit.

The power detection circuit output signal VDET is generated at terminal32 by passing the current in transistor 54 through resistor 62, and acapacitor 64 is provided in parallel with resistor 62 to create a simpleRC averaging circuit (although other types of averaging circuits may beemployed) so that the output V_(DET) represents an averaged signal whichmore accurately represents the power level in the amplifying transistor.

In this manner, the present invention provides a power detection circuitfor a power amplifier circuit suitable for use in RF or audioamplifiers, in which an output is generated which is directlyproportional to the power level in the amplifying transistor. Circuitsin accordance with the invention are compact, efficient, easilyimplemented and provide a signal which more accurately represents thepower level in the amplifying transistor than do prior-art circuits.

While the invention has been particularly shown and described withreference to several preferred embodiments thereof, it will beunderstood by those skilled in the art that various changes in form anddetail may be made without departing from the spirit or scope of theinvention. Thus, for example, different types of power amplifyingcircuits, sensing circuits, squaring circuits and averaging circuits maybe employed as appropriate, and alterations to the circuitconfigurations shown may be made to suit particular design requirements.

What is claimed is:
 1. A power amplifier circuit for amplifying an inputsignal and comprising an amplifying transistor and a power detectioncircuit, said power detection circuit comprising a circuit forgenerating a signal which is directly proportional to a power level insaid amplifying transistor, wherein said circuit for generating a signalcomprises a circuit for generating a voltage proportional to the squareof a current in said amplifying transistor.
 2. A power amplifier circuitas in claim 1, wherein said circuit for generating a signal furthercomprises a circuit for averaging said voltage proportional to thesource of the current in said amplifying transistor.
 3. A poweramplifier circuit as in claim 1, wherein said circuit for generating asignal comprises a sense t ransistor having a control terminal connectedin parallel with an input terminal of said amplifying transistor and anoutput terminal, and an MOS transistor having a control terminal coupledto said output terminal of said sense transistor and an output terminalfor providing said signal which is directly proportional to the powerlevel in said amplifying transistor.
 4. A power amplifier circuit as inclaim 1, wherein said circuit for generating a signal comprises a sensetransistor having a control terminal connected in parallel with an inputterminal of said amplifying transistor and an output terminal, and atranslinear circuit comprising bipolar transistors and having an inputterminal coupled to said output terminal of said sense transistor and anoutput terminal for providing said signal which is directly proportionalto the power level in said amplifying transistor.
 5. A wirelesscommunications device having an RF power amplifier circuit foramplifying an input signal and comprising an amplifying transistor and apower detection circuit, said power detection circuit comprising acircuit for generating a signal which is directly proportional to apower level in said amplifying transistor, wherein said circuit forgenerating a signal comprises a circuit for generating a voltageproportional to the square of a current in said amplifying transistor.6. A power detection circuit for use in a power amplifier circuit foramplifying an input signal and comprising an amplifying transistor, thepower detection circuit comprising a circuit for generating a signalwhich is directly proportional to a power level in the amplifyingtransistor, wherein said circuit for generating a signal comprises acircuit for generating a voltage proportional to the square of a currentin said amplifying transistor.